An area flooded by the Bourbeuse River in Missouri

Mandatory and voluntary evacuations are underway for parts of two Midwestern U.S. states of Missouri and Illinois as historic flooding has killed at least 20 people in the two regions.

Missouri Governor Jay Nixon has ordered the National Guard, a state-based military force, to help route traffic at blocked highways and roads, along with providing security to residents. The U.S. Army Corps of Engineers issued a statement saying that 19 federal levees in the St. Louis area are at risk of being overtopped.

Two towns near the Missouri city of St. Louis, about 10,000 people in total, have been ordered to evacuate by their mayors, after a levee failed along the Mississippi River, and a sewage plant also malfunctioned and failed.

In a statement, the U.S. government’s National Weather Service warned that the flood stage for the Mississippi River at St. Louis may reach 42.5 feet, or about 12.95 meters, by 11 a.m. local time Thursday, which would be the second-worst flood on record, behind the Great Flood of 1993, which killed 32 people and caused 15 billion U.S. dollars in damage.

Airlines in the O’Hare International Airport in Chicago, Illinois, cancelled more than 150 flights and delayed 130 more Wednesday, according to the airport’s website.

The U.S.-based FlightAware website, which pulls data from the U.S. Federal Aviation Administration and global airlines, showed that 22 flights bound for China originating from the Chicago O’Hare airport were delayed. Additionally, a United Airlines flight from Seattle to Chicago’s O’Hare International Airport rolled off the runway Wednesday morning. Although there were no injuries, the plane had to be towed.

The worst is yet to come for the Midwestern U.S., as the National Weather Service shows that Mississippi River is not set to reach its peak until Thursday.